This invention relates to a display device comprising a first substrate having at least one transparent, first picture electrode of a first material, a second substrate having at least one second picture electrode of a second material which, jointly with the picture electrode on the first substrate and an intermediate opto-electronic material, defines a pixel, and means for supplying electric voltages to the picture electrodes.
Display devices of this type are generally known and usually comprise a large number of pixels. The first type comprises, for example, LCDs, both of the passive and active type; the second type comprises, for example, plasma-addressed liquid crystal display devices referred to as PALC displays.
Notably when the second picture electrode is reflective, the drive of these display devices appears to be sensitive to the alternating supply of positive and negative voltages across the pixels. This becomes manifest as, for example, flicker. In a picture period, in which a positive voltage is written, the same picture information leads to a different voltage across the pixel than in the subsequent picture period in which a negative voltage is written. At a frame frequency of, for example, 60 Hz, this leads to a flicker frequency of 30 Hz, which is clearly noticeable in the picture. The display devices also often suffer from image retention. In this application, reflecting picture electrodes are also understood to mean partially reflecting, semi-transparent electrodes, as are used in transflective display devices.
An object of the invention is to provide a display device that is at least partly or entirely free of one or more of said problems.
Another object of the invention is to provide a display device in which the flicker problem is substantially reduced or eliminated.
These and other objects are accomplished, according to a first embodiment of a display device of the invention in which at least one of the picture electrodes is coated with at least one layer of a passivating material such that any asymmetry in operation of the device is substantially reduced or eliminated.
In preferred embodiments of the invention, at least one of the picture electrodes is coated with at least one layer of a dielectric material, and at least one layer of a passivating material such that any asymmetry in operation of the device is decreased.
Most preferably, the reflective LCDs of the invention may utilize a silicon chip as the reflective wall of the cell and a glass sheet as the transmissive wall. The addressing electronics are integrated on the chip, along with the matrix electrodes and switches. Aluminum metalization for the electronics also serves as the reflective electrode of the cell, while the standard indium tin oxide (ITO) serves as the transmissive electrode. A passivating material is provided on the entire silicon chip or a portion thereof with the effect that any asymmetry in operation of the device is substantially reduced or eliminated.
As used in this invention, the term xe2x80x9cpassivating materialxe2x80x9d and its derivations means a material effective to modify or otherwise adapt the materials comprising the device to exhibit a reflective response having a degree of asymmetry that is decreased when compared to the asymmetry of the same device in the absence of such material.
This invention is based on the recognition that the xe2x80x9cflickerxe2x80x9d problem is a result of cell asymmetry. We have observed that the reflective LCD cell or pixel, when operated with a symmetric ac electrical signal, develops in time an asymmetric reflection response. While not intending to be bound by any one theory or hypothesis for the invention since there may be several possible causes for the occurrence of this phenomenon, we have found that the reflection asymmetry arises due to the asymmetry of the materials that make up the cell, i.e. the use of materials that are dissimilar in certain electrical properties or functions which leads to asymmetry of the device in operation. In our work, the reflective LCD cell may be analogized to a leaky capacitor. A typical construction includes one electrode comprising a reflective picture electrode, for example formed of aluminum, and a second electrode comprising a transparent picture electrode, for example formed of ITO. A dielectric material, for example, a polyimide dielectric, is present in intimate contact with each electrode, and a liquid crystal material connects the two layers. One of the central premises of this invention is that the Schottky barrier formed between the aluminum electrode and polyimide dielectric is fundamentally different from the Schottky barrier formed between the ITO electrode and polyimide dielectric. This is thought to be caused by the different material properties of the two electrodes which are believed to result in the leakage in ac operation being asymmetrical. It is believed that this in turn leads to an excess charge and thereby to an asymmetric electric field in response to a symmetric voltage. It is also believed that the local electric field affects or governs the LC response rather than the voltage on the remote electrodes, and that the invention reduces the asymmetry by reducing the dissimilarity between the materials, for example the aluminum and ITO, by applying a layer of a material effective to passivate the aluminum or ITO or both, preferably a layer of a noble metal such as Au, Pt, Ir, Rh, or Pd, most preferably Au, on the aluminum, or on the ITO, or both. Due to the stepped profile that is normally present in the silicon chip, the noble metal layer is discontinuous, and thus does not short out the aluminum metalization.
Regardless of the actual mechanism or hypothesis by which the invention works, it has been found that the application of such a passivating layer or passivating material is effective to substantially decrease or eliminate the xe2x80x9cflickerxe2x80x9d problem in the display devices that contain such passivated reflective LCDs that result from the invention.
This invention also relates to a method for producing such passivated reflective LCDs which comprises the steps of providing a passivating material, such as a noble metal, preferably Au, on a surface of the device, preferably adjacent the metalization or on or adjacent a portion of the device that comprises the metalization.
The method may be accomplished in different ways, for example, by coating the transparent picture electrode or the reflecting picture electrode, or both, with at least a layer comprising at least a thin layer of the same or different passivating material.
Also, when using a (organic) dielectric material, which is preferably chosen from the group of polyimides or polyamide acids comprising or not comprising fluorine, the reflecting picture electrode and the transparent picture electrode may be coated with a layer comprising at least a layer of the same (organic) dielectric material, the material on one of the two picture electrodes having been subjected to, for example, a UV treatment when these electrodes are coated with a layer comprising at least a layer of the same (organic) dielectric material.
If desired, the layer of dielectric material may function as an orientation layer.
Specifically, in its most preferred embodiments, in the display devices of the invention, the passivating material will be a thin layer of gold, platinum, iridium, rhodium, or palladium provided on a silicon wafer by a method which comprises coating the entire silicon wafer as received from a silicon fabrication process, with a thin layer of passivating material. The starting silicon chip for example, will comprise a surface on which reflecting aluminum electrodes may be provided, while switching elements (transistors) may be provided in the subjacent silicon.
Also in especially preferred embodiments, a reflective LCD is made using one ITO electrode, and using the aluminum metalization on the silicon chip as the other LCD electrode, the full addressing electronics being implemented in the silicon chip, the aluminum serving as the reflector as well as the metalization. In such embodiment, the passivating material is preferably gold (Au) provided between the aluminum on the silicon chip and the rest of the LCD cell.
Various amounts of the passivating material may be used, with preferred amounts ranging from about 1 nm to about 5 nm. It is important that the passivating material be present in amounts that result in thin films, so that when applied, it does not cause a short circuit among the many separate pixels of the display. We have found that when Au is the passivating material present in amounts that give rise to thicknesses from about 1 nm to 5 nm, the thickness of Au present cannot form a continuous film bridging otherwise separate Al metal regions, due to the thickness of the Al, and the resulting surface texture. Thus, it is possible to take advantage of the step coverage to avoid shorting out the circuit.